What you do is go back to your therapist and take your letter with you. You’ve written an articulate and heart felt description of your feelings and confusions. Reading it together will help your therapist better understand what you are dealing with.
You lost a brother and your parents lost a son. It’s not at all unusual for grieving to go on for a long time and to take many forms from self-destructive behaviors to depression to anger. Unresolved grieving often puts a family in turmoil as different people deal with it in different ways. In your case, I’m guessing that it’s difficult for you to move on and have the life that your brother will never have.
Please take care of yourself. Medication may help some but grief isn’t something you can medicate away. Go back to therapy. If that therapist isn’t responsive, then find another therapist who specializes in grief. You — and your family — deserve the help.
I wish you well.